Problem: Even though 'Related changes' can show up to 500 changes, it has a short lifespan, just 30 days maximum, so rarely shows more than 20-30 before they time-expire. 'Related changes' uses the recentchanges table, which for performance reasons on Wikimedia cannot be increased from 30 days worth of data.
Who would benefit: Editors checking for changes on related pages.
More comments: It is very easy to forget to check 'Related changes' monthly. Extending it to a year would allow editors to keep a more relaxed overview of checking changes on related pages without worrying if they've missed anything important.